The metabolism of certain carcinogens by the intestinal bacteria may determine the effect of these compounds in the host. It is known for example that the flora are responsible for the hydrolysis of the glucuronides of diethylstilbesterol and N-hydroxy-fluorenylacetamide (N-OH-FAA) and that the flora are capable of the N-dehydroxylation of N-OH-FAA. These reactions may be significant since the glucuronide of N-OH-FAA and the product of N-dehydroxylation of N-OH-FAA (fluorenylacetamide) are less carcinogenic than N-OH-FAA. The flora of rats is altered in response to the feeding of N-OH-FAA and the flora is different in human populations with different incidences of cancer. The role of the individual components of the intestinal microflora in mediating reactions of possible significance in carcinogenesis is largely unknown; hence alterations in the composition of the intestinal flora cannot be related directly to the incidence of cancer. Accordingly it is proposed to characterize the predominant cultivable flora of the rat and to determine the reactions of these bacteria with certain carcinogens. The bacteria which transform these carcinogens can then be associated with germ-free rats to determine if metabolic transformation lacking in the germ-free rat can be found in the resulting gnotobiotic rat. Using these rat models it will be possible to determine the role in carcinogenesis of these transformations by the intestinal flora of the host.